A styrene-divinylbenzene porous resin particle has been widely used in the fields of ion exchange resin, adsorbent, packing agent for liquid chromatography, support for solid phase synthesis, and the like. The porous resin particle used for such applications has some functional group in many cases and is usually produced though a plurality of steps. For example, there are a method including preparing a styrene-divinylbenzene porous resin particle by suspension polymerization, collecting the obtained resin particle, and reacting various substances with the residual vinyl group to introduce a functional group into the resin (see, for example, Patent Documents 1 to 3). According to such a method, however, two steps, that is, polymerization step and functionalizing step, are necessary.
Furthermore, there is also a method of grafting a polyethylene glycol to a polychloromethylstyrene resin to produce a porous resin particle having a hydroxy group (see, Patent Document 4), but this method also requires two steps, that is, polymerization step and grafting step.
As another problem, the styrene-divinylbenzene porous resin particle is well swelled in a nonpolar solvent such as toluene and dichloromethane but is difficult to swell in a polar solvent such as acetonitrile and ethanol. Particularly, in a solid phase synthesis reaction using a styrene-divinylbenzene porous resin particle as the support for solid phase synthesis, for obtaining the objective reaction product in a larger amount, the porous resin particle is preferably swelled to a certain extent in an organic solvent. However, in the case where a chemical reaction is performed by sequentially using a plurality of organic solvents and where the swelling degree of the resin differs among the organic solvents, when, for example, a column-like reaction vessel having a given volume is used, the pressure fluctuates in the vessel and this may give rise to variation in the yield of the objective synthesis reaction.    Patent Document 1: U.S. Pat. No. 3,586,644    Patent Document 2: U.S. Pat. No. 4,098,727    Patent Document 3: U.S. Pat. No. 4,950,408    Patent Document 4: U.S. Pat. No. 4,908,405